Such a driver brake back-feeding pre-steering servo devices are provided for the steering of a servo medium which may be either pneumatic or electric. A valve unit (analog converter) converts the appropriate signal level of the servo medium to a corresponding servo pressure, which controls a relay valve, which directs the pressure in the main air line corresponding to the servo pressure.
The servo devices (input units) are, in this case, formed as pure electrical switches whose output signals affect the magnet valves of the analog converters directly. In order to be able to set the servo pressure in a non-linear manner from the activation times of the electrical switches for the various procedures such as braking, releasing, adapting, injection, etc., the known analog converter has elaborate pneumatic devices similar to those of the known purely pneumatic driver brake valves (Cf. house publication of KnoorBremse GmbH: "Fuhrerbremsventilanordnung HDR fur Streckenlok," 1974 edition - "HDR Drive Brake Valve System for Long-Distance Locomotives") (TK3 0022a). From DE-A-31 49 110, an electro-pressure-medium braking system for directly-operating brakes of road vehicles is known, in which the brake medium pressure is modified directly by means of magnetic valves controlled by electrical signal emitters. This brake system is not suitable for automatic indirectly-operating air-pressure brakes on rail vehicles.
Finally, a regulating switch system for aircraft brakes is known from EP-B1-0026725, in which an electronic regulating switch directly affects electromagnetic valves, which directly change the brake medium pressure. This system, too, is unsuitable for indirectly operational brakes.